Method of refining a carbonaceous metal



March 3, 1970 F. K. E. JoHANssoN- EFAL 3, 3,

METHOD OF REFINING A CARBONACEOUS IETAL Filed May 4,1967

IIIVEITORS FOLKB- KARL EVALD JOIIAISSOI G'OSTA HUGO CBDBBVALL M MvW ATTORNEYS United States Patent 3,498,783 METHOD OF REFINING A CARBONACEOUS METAL Folke Karl Evald Johansson, Borlange, and Gosta Hugo Cedervall, Falun, Sweden, assignors to Stora Kopparbergs Bergslags Aktiebolag, Falun, Sweden, a Swedish company Filed May 4, 1967, Ser. No. 636,159 Int. Cl. C21c 7/04 U.S. CI. 7560 20 Claims ABSTRACT OF THE DISCLOSURE The invention refers to refining pig iron, or another carbon-containing metal by blowing an oxygen-rich gas against a bath of the metal and silicious slag in a furnace having silicious lining and means for moving the bath relative to the lining. Gas and slag content of the produced metal is depressed by passing combustion heat from the gas space above the bath to the lining and from the lining to the pig iron of high temperature and sufficient carbon content to release silicon from the lining, and slag.

The present invention relates to the refining of a carbonaceous metal bath, particularly but not exclusively, molten pig iron and other molten carbonaceous alloyed or unalloyed iron materials, the refining being carried out by blowing a gas rich in oxygen against the bath in a refractory-lined furnace provided with means for producing a relative motion between the bath and the lining.

The primary object of the invention is to provide a new and advantageous method of producing extremely clean metal, particularly steel. The object of the invention is also to produce a metal with a low content of dissolved gases, e.g. hydrogen, thus avoiding small, interior cracks, so-called flakes, and other defects derived from gases in the metal.

The refining of carbonaceous metal 'baths by blowing a gas rich in oxygen against the bath in refractory-lined furnaces is well-known in the art. The relative motion between the bath and the lining can be produced by various means e.g. electromagnetic, pneumatic or mechanical means, and the use of them in the refining of metal baths has been earlier proposed.

According to the invention the gas and slag content of the refined metal is lowered in the following way: A substantial part of the heat generated in the furnace is transferred to the bath by the furnace lining. The furnace lining used is rich in silica. The slag formed in the process is kept rich in silica, but also contains iron and possibly other oxides to make it lightly fluid. At some, preferably late, stage of the refining, the silica content in the lining is reduced by the carbon in the bath and the silicon thus produced is dissolved in the metal bath until it contains between 0.05 and 0.5% Si by weight.

The reduction of silica according to the formula 2C+Si0 =2CO+Si is endothermic and is promoted by a high temperature and a high carbon content in the metal. The temperature at which the reaction starts is the higher the lower the carbon content of the metal. The reaction occurs essentially at the interface between the lining and the metal bath. In view of this the conditions for said reaction are particularly favourable in furnaces, wherein heat is conducted to the bath through the furnace lining, the lining thus having a higher temperature than the metal bath.

Of course, the heat may be generated outside the furnace and conducted through the lining into the bath, but it is more convenient and improves the thermal efficiency if the heat is generated inside the furnace, preferably by burning the carbon monoxide which is generated by the oxidati n of carbon in the melt with oxygen and oxides in the slag and in the lining. In order to transfer this heat from the lining to the bath, the lining suitably is moved in such a way that it alternatingly comes into contact with the burning gases and the bath.

In accordance with the invention, it has been possible to product a steel with a very low slag content. The slag content of the steel can be determined by measuring the oxygen content of the steel. As the oxygen content also depends on the carbon content, the product of carbon content in percent and oxygen content in p.p.m. (parts per million) may be used as an indication of the slag content. According to the invention it has been possible to obtain 28 p.p.m. oxygen at 1.04% carbon, said product being 29, which is a very good result in the art.

A slag poor in lime but rich in silica is able to absorb hydrogen from the surrounding atmosphere only to a minor extent. By operating with a slag of the type indicated the absorption of hydrogen in the steel and consequently the formation of flakes is avoided to a great extent. Moreover, contributive to the low hydrogen content of the acid steel is also the fact that CO-bubbles formed by the reduction of silica at the interface between metal bath and lining rise through the bath and wash out hydrogen that may be present in the bath.

The invent on is conveniently carried out in two or three stages. In a first stage of the slag-forming elements and a substantial part of the carbon in the metalis oxidized at temperatures lower than that required for starting the silica reduction at the actual carbon content. In a second stage the refining is continued with an increase in the temperature to such a level that the silica reduction starts. The slag and gas contents may be further reduced in a third stage in which there is essentially no blowing of gas rich in oxygen and relative motion between the bath and the lining.

While according to the invention any type of furnace can be used, in which heat is transferred to the bath by the furnace lining, the object of the invention is best obtained by using a rotary furnace, particularly a furnace wherein the furnace lining is heated in the gas space and releases heat when coming into contact with the molten bath during the rotation.

Therefore, in the following, from which further objects and features of the invention will appear, the method of the invention will be described in connection with a rotary furnace with reference to the appended drawing.

The rotary furnace shown is of the type known per se used in the Kaldo-process for steel manufacture by refining molten pig iron with a gas rich in oxygen. The furnace is provided with means for supplying the gas rich in oxygen into the space above the bath for .oxidation of the carbon of the bath into carbon monoxide and combustion above the bath of the carbon monoxide thus generated into carbon dioxide. By rotating the furnace about a horizontal or inclined axis, a part of the heat generated by the combustion of the carbon monoxide and absorbed by the furnace lining can be transferred to the bath when the lining, due to the rotation of the furnace, gradually passes down below the surface of the bath.

According to an embodiment of the present invention the furnace l, which is provided with a lining 2 rich in silica, preferably having a SiO content above by weight, is charged with a metallic starting material consisting of molten pig iron. The furnace is surrounded by a frame 2 with rings resting on rollers 5, 6. The roller 5 is driven by a motor 7, whereby the furnace can be rotated about its axis. In operation a gas rich in oxygen can be blown through the lance 12 into the space above the bath. The oxidation with free oxygen of the carbon and slag forming elements usually present in the metal, such as silicon, manganese, and a small part of the iron, takes place exothermally. The fraction of the heat released, which is not required to increase the temperature of the bath and to compensate for heat losses to the surroundings and the exhaust gases, can be used for melting Fe-containing material, suchas scrap, iron ore and prereduced ore, in order to increase the amount of steel produced per ton of pig iron charged.

The additions may be supplied before starting the blowing, at a stop in the blowing and/or during the supply of the gas rich in oxygen. The quantity of cooling agents which can be added is determined by their cooling, capacity, the temperature and analyses of the pig iron and the steel produced, the composition of the supplied gas, the dimensions of the furnace and the time interval between the heats. The gas rich is oxygen may consist of oxygen enriched air having an oxygen content of at least 40% by volume and consists advantageously of com mercially pure oxygen gas, having an oxygen content above 95% by volume. A slag rich in FeO is formed on top of the bath. This slag together with further iron oxide, which may be formed if the iron comes into direct contact with the oxygen gas, reacts with carbon in the bath under formation of carbon monoxide, which is burnt into carbon dioxide with oxygen present in the atmosphere above the bath. The heat generated thereby is largely absorbed by the lining. By rotation of the furnace the lining transfers the absorbed heat to the bath.

Initially, the refining is carried out while maintaining a relatively vigorous relative motion between bath and lining by rotating the furnace at at least r.p.m. but below the speed at which the centrifugal forces hinder the relative motion between bath and lining.

In order to obtain a fast refining it is necessary to have a lightly fluid and reactive slag, which presupposes that it is unsaturated with regard to SiO The fluidity of the slag can be increased by adding lime. However, it should be observed that such an addition should be limited in order to prevent the furnace lining from being attacked by the slag and to avoid a tendency of the slag to absorb hydrogen. Thus, the lime should not exceed about 10% by weight .of the slag and is suitably maintained at about 3%.

In order to avoid too large a content of FeO in the slag before the temperature is high enough for a reaction between the FeO and the carbon in the bath, it may be suitable to introduce a carbon-containing fuel with low content of phosphorus and sulphur, e.g. graphite or methane, into the furnace.

After this first stage, in which a great part of the carbon, all silicon and other elements that may be present in the melt have been oxidized, the slag is partly tapped and the temperature and composition of the melt is determined. At this stage the carbon content of the metal is conveniently about 0.51% above that wanted in the end analysis, the temperature of the melt is between 1500-1600" C., and the FeO-content of the slag just high enough to make the slag fluid.

If the charge material is low in manganese, and thus the MnO content in the slag is low, some manganese containing slagformer may be added before the refining is continued in a second stage. Also alloying elements, e.g. chromium, may now be added. These additions may be supplied in elementary form .or as compounds reducible by carbon at the prevailing temperatures, which may be 1600 C. or more. The additions of cooling agents and oxygen are controlled in such a manner that the temperatures of the furnace lining and the melt become high enough to start the silica reduction at the actual carbon content of the melt.

The refining is continued concurrently with the r duction of silica until the melt has the intended temperature and contents of carbon and silicon.

The reduction of silica is promoted by a high temperature of the lining. This condition can be obtained when using a rotary furnace at the speed of rotation mentioned above but may be further improved by gradually or promptly decreasing the speed of rotation. However, a moderate relative motion should be maintained and it is suitable to maintain a speed of rotation of the furnace of at least 5 r.p.m.

In many cases it has been found suitable, in a third stage, to proceed with practically no blowing of oxygen and no rotation of the furnace when the carbon content exceeds the intended final carbon content of the metal by about 0.05% by weight of the metal bath. Especially in some cases, where the oxygen content of the slag and metal at the end of the refining is high, a stand-still in a few minutes can decrease the oxygen content without changing the Si content.

In some cases it has been found suitable before the third stage to first maintain a vigorous relative motion between the bath and the lining until the bath is refined to a carbon content, which exceeds the intended final carbon content by about 0.2% of the weight of the metal, and thereafter to maintain a moderate relative motion between the bath and the lining until the carbon content exceeds the intended final carbon content by about 0.05% by weight.

To prevent the part of the furnace lining extending above the bath during the stand-still from losing its heat by radiation, the mouth of the furnace is suitably kept closed during the reduction of silica. Possibly, the carbon monoxide formed at this reduction of silica can be oxidized to carbon dioxide by carefully injecting a small amount of oxygen gas during the stand-still in order to reduce the thermal loss. If, instead, the lining above the bath is allowed to lose part of its heat so that its temperature comes below that needed for reduction of silica, the reaction can be stopped at an appropriate moment by moving this part of the lining into contact with the bath.

The third stage may further be modified in such manner that an inert gas, e.g. argon, is introduced into the furnace, while maintaining a relative motion between the bath and the lining.

At the end of the process carried out according to the invention, determinations of C and Si are carried out in a manner known per se, the temperature also being recorded in a manner known per se. Before the tapping, necessary amounts of alloying compounds may be added, for instance in the form of FeS'i, FeMn and for FeCr. At least when added in major amounts, said compounds act as cooling agents, and when added in sufliicent amounts they will thus interrupt the reduction of silica. When carrying out the tapping over lip, it is advantageous that the slag is stiffened in the region close to the mouth of the furnace, for instance by charging quartz or crushed, cooled slag from previous heats to said region, thereby causing the stiffened slag to form a bridge at the mouth of the furnace preventing the slag of a higher fluidity present behind the bridge to accompany the steel during the tapping.

In some cases it may be desirable to use at least a part of the slag in a subsequent heat. This may be the case, for instance, when using metal having a small content of silicon yielding an insuflicient amount of silica in the slag. Therefore, when using such types of metal it may be desirable to use the same slag in a number of subsequent heats. Particularly in such an embodiment of the invention it may suitable to have the possibility to tap the steel through a tap hole positioned in the bottom 1 wall of the furnace, which tap hole suitably is closed during the rotation of the furnace. The tapping of the steel through the hole is interrupted when the slag appears.

The following example illustrates a specific embodiment which falls within the scope of this invention:

EXAMPLE A charge comprising 17 tons of hot pig iron of a temperature of 1220 C. and 3.1 metric tons of rolling mills scrap having a low content of phosphorus and sulphur was introduced into a Kaldo-furnace with a capacity of 25 tons. The furnace was lined with silica bricks, and the surface temperature of the lining was about 1000 C., remaining from the preceding heat. The furnace also contained the final slag from said heat.

In a first stage the pig iron was refined by blowing commercial pure oxygen through a water cooled lance with an inclination of 20 against the horizontal. After 13 minutes, when 520 m. oxygen had been blown into the furnace, a further quantity of 3 tons of scrap was added. During the next 15 minutes 480 m. oxygen were blown into the furnace, which was rotated during both periods of blowing at a speed of 26 r.p.m.

At the end of the second blowing period all scrap was molten. The temperature of the steel bath was 1560 C., and a first sample of the steel was analyzed. Most of the slag, which comprised both the slag from the preceding heat and the slag formed during the refining, was tapped and 100 kg. of ferro-manganese were added in order to increase the MnO-content of the slag left in the furnace.

From the carbon content of the first steel sample the oxygen amount needed for the continued refining in the second stage was calculated. The refining was completed by blowing 190 m. oxygen against the bath during a total period of 7 minutes. The furnace was now rotated with a speed of 20 r.p.m. After this blowing period the temperature of the steel bath had risen to 1640 C. After having taken a second sample on the steel the furnace in the third stage was left in horizontal position and the mouth was closed with a lid, which was lined with refractory bricks. A boil caused by the reaction of the carbon in the bath with the silica in the lining took place. 12 minutes after the second steel sampling a third steel sample was taken. The temperature of the bath was now 1615 C. and the boil had almost ceased.

Alloying elements were added and thereafter the slag in the mouth was stiffened by coarse quartz in order to get a darn, which kept the slag in the furnace while the steel was tapped in a ladle. The analyses of the metal bath has been compiled in the following table.

Percent Pereent Percent Percent Percent Analyses C Si Mn P S Pig iron 4. 10 0. 21 0. 03 0. 011 0. 004 Steel sample:

What is claimed is:

1. In refining a carbonaceous metal bath by blowing a gas rich in oxygen against the bath in a refractory lined furnace provided with means for producing a relative motion between the bath and the lining of the furnace, the method of lowering the gas and slag contents of the metal bath which comprises, transferring in an earlier stage a substantial part of the heat generated within the furnace to the lining and from the lining to the bath, said furnace lining and slag being rich in silica and increasing the temperature to above about 1600" C. to reduce the silica content in the lining by reaction with carbon in the bath in a late stage of the refining, and dissolving the silicon thus produced in the metal bath until the bath contains between 0.05 and 0.5% Si.

2. A method according to claim 1 wherein the carbon monoxide generated at the oxidation of the carbon in the melt by oxygen and oxides in the slag and in the lining is burnt within the furnace and the heat thereby generated is transmitted to the bath by the lining because of the relative motion between the bath and the lining.

3. A method according to claim 1, wherein the furnace lining initially contains at least 75% SiO 4. A method according to claim 1, wherein the slag contains less than 10% CaO, preferably less than C210.

5. A method according to claim 1, wherein the furnace is a rotary furnace the axis of which is horizontal or is inclined not more than 45 to the horizontal.

6. A method according to claim 1, wherein, in a early stage, slag-forming elements and a substantial part of the carbon in the metal bath are oxidized by the gas rich in oxygen at temperatures below those needed for starting the silica reduction and, in a later stage, the refining is continued at a temperature above about 1600 C.

7. A method according to claim 6, wherein slag forming materials containing oxides of silicon, and calcium are added in the first stage.

8. A method according to claim 6, wherein fuel is added in the first stage.

9. A method according to claim 6, wherein the temperature in the first stage is kept below 1600 C.

10. A method according to claim 6, wherein a part of the slag formed in the first stage is removed.

11. A method according to claim 6, wherein slag-forming materials containing manganese are added between the first and the second stage.

12. A method according to claim 6, wherein alloying elements are added between the first and the second stage.

13. A method according to claim 12, wherein the alloying elements are added in the form of oxides that are reducible by carbon at temperatures above 1600 C.

14. A method according to claim 6, wherein the desired final contents of carbon and silicon in the metal bath are established in an additional stage in which there is essentially no relative motion between the bath and the lining.

15. A method according to claim 14, wherein the third stage is started before the carbon content of the metal bath has reached a value of about 0.05% by weight above the desired final carbon content.

16. A method according to claim 14, wherein the third stage begins when the temperature of the bath exceeds the intended tapping temperature, suitably by a value of at least 20 C.

17. A method according to claim 14, wherein the bath in the second stage, while maintaining a vigorous relative motion between the bath and the lining, is refined to a carbon content, which by a value of about 0.2% by weight exceeds the intended final carbon content, and that the bath then, while maintaining a moderate relative motion between the bath and the lining, is further refined to a carbon content, which by a value of about 0.05% by weight exceeds the intended final carbon content.

18. A method according to claim 14, wherein the fur nace is essentially closed after the ceasing of the relative motion between the melt and the lining.

19. A method according to claim 14, wherein the carbon monoxide generated at the silica reduction during the third stage is burnt inside the furance.

20. A method according to claim 1 wherein an inert gas, preferably argon, is introduced into the furnace during the last part of the silica reduction period while maintaining a relative motion between the bath and the lining.

References Cited UNITED STATES PATENTS 2,853,377 9/1958 Kalling et al. -60 2,883,279 4/1959 Graef et al. 75-59 X 2,930,688 3/1960 Kalling et al 75-60 X 3,034,885 5/1962 Hardt Qt 75-60 X 3,307,937 3/1967 Pihlblad et al. 75-60 X HYLAND BIZOT, Primary Examiner G. K. WHITE, Assistant Examiner US. Cl. X.R. 

